ARTICLE
Received 9 Dec 2014 | Accepted 24 Apr 2015 | Published 15 Jun 2015
DOI: 10.1038/ncomms8291
OPEN
Ambient solid-state mechano-chemical reactions
between functionalized carbon nanotubes
Mohamad A. Kabbani1, Chandra Sekhar Tiwary1,*, Pedro A.S. Autreto2,*, Gustavo Brunetto2,*, Anirban Som3,
K.R. Krishnadas3, Sehmus Ozden1, Ken P. Hackenberg1, Yongi Gong4, Douglas S. Galvao2, Robert Vajtai1,
Ahmad T. Kabbani1,5, Thalappil Pradeep3 & Pulickel M. Ajayan1,3,4
Carbon nanotubes can be chemically modified by attaching various functionalities to their
surfaces, although harsh chemical treatments can lead to their break-up into graphene
nanostructures. On the other hand, direct coupling between functionalities bound on
individual nanotubes could lead to, as yet unexplored, spontaneous chemical reactions.
Here we report an ambient mechano-chemical reaction between two varieties of nanotubes,
carrying predominantly carboxyl and hydroxyl functionalities, respectively, facilitated by
simple mechanical grinding of the reactants. The purely solid-state reaction between the
chemically differentiated nanotube species produces condensation products and unzipping of
nanotubes due to local energy release, as confirmed by spectroscopic measurements, thermal analysis and molecular dynamic simulations.
1 Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, USA. 2 Department of Applied Physics, State University of
Campinas, Campinas-SP 13083-959, Brazil. 3 DST Unit of Nanoscience and Thematic Unit of Excellence, Department of Chemistry, Indian Institute of
Technology Madras, Chennai 600 036, India. 4 Department of Chemistry Rice University, Houston, Texas 77005, USA. 5 Department of Natural Science,
Lebanese American University, P.O. Box 13-5053 Chouran, Beirut 1102 2801, Lebanon. * These authors contributed equally to this work. Correspondence and
requests for materials should be addressed to M.A.K. (email: [email protected]) or to T.P. (email: [email protected]) or to P.M.A. (email: [email protected]).
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8291
C
hemical functionalization of nanoparticles can lead to
their surface decoration with a variety of covalently
attached functionalities to serve different goals such as
drug delivery, cancer therapy, diagnostics and electronic
devices1–6. Carbon nanotubes (CNTs) have been the subject of
more than two decades of intense research7–10. Various
approaches have been used to modify their surfaces via
covalent and non-covalent attachments to change both physical
and chemical properties. Activation of CNTs by the incorporation
of COOH on the exterior surfaces by treating them with
concentrated nitric acid has been widely used11,12. CNT-COOH
can further be activated by acylation to form CNT-COCl, which
can in turn be amidated or esterified13,14. CNTs carrying
hydroxyl groups (CNT-OH) on their surfaces have also been
synthesized by alkaline hydrothermal treatment of pristine
nanotubes in alkaline medium15. Despite the remarkable CNT
mechanical and electronic properties, their large use has been
precluded by poor solubility in water or organic solvents, which
favours bundle formation, thus limiting their chemical reactivity.
Mechano-chemical reactions (MCRs) can be used to overcome
such difficulties16,17. MCRs have been extensively used as
synthetic protocols to obtain fullerene derivatives. Examples of
these methods are fullerene dimers (C120), trimers (C180), cross
dimers (C60-C70) and other fullerene derivatives obtained by the
solid-state reactions with potassium salts such as KCN, K2CO3,
reducing metals such as Mg and Al and solid aromatic amines
under the high-speed vibration milling conditions18–21. MCRs
of CNT functionalizations with other molecules such as C60,
nitrenes, diazoinium compounds and metallic hydroxides under
vigorous milling conditions have also been reported22–25.
Another way of reacting fullerenes has been through their
encapsulation into CNTs26. On the other hand, many methods
have been used to produce graphene27–31, including the
unzipping of nanotubes to make graphene nanoribbons. A
typical chemical unzipping of CNTs makes use of oxidative
techniques31 in concentrated acid (H2SO4) and post treatments
with harsh reagents such as highly concentrated potassium
permanganate (KMnO4). These processes involve harsh
conditions to get to the final product (graphene), which can
contain broken up or unzipped CNT. What has not been done is
the use of nanotubes as solid-state reaction templates with specific
chemical surface functionalities to induce direct coupling between
the functional groups and concomitant breakdown of the
cylindrical structure.
Here, we report the demonstration of unzipping of CNTs via a
solid-state room temperature reaction between multiwalled CNTs
(MWCNTs) containing different reactive functionalities of
COOH and OH groups. The reaction is mechano-chemically
induced, initiated at room temperature in ambient air, facilitated
by the simple grinding of two chemically variant CNT reactants
and leading to the unzipping of the nanotube (shown in Fig. 1a).
By grinding equal weights of MWCNT-COOH and MWCNTOH decorated with 1.41% and 0.46% by weight of COOH and
OH (see experimental details of grinding and Supplementary
Table 1), respectively11,15, a sheet-like lustrous material is formed
spontaneously (Fig. 2a). Characterization of the material using
different microscopic and spectroscopic methods, described later
in the manuscript, suggests that the product consists
predominantly of graphene or partially opened CNTs, possibly
formed via the unzipping of the MWCNT reactants. The
unzipping reaction may be represented by equation (1)
MWCNT COOH þ MWCNT OH!!G þ G; þ H2 O þ CO2 ð1Þ
Where G and G0 represent the graphenes originating from the
carboxylic and hydroxyl MWCNT (functionalized MWCNTs).
2
Results
Characterization. Attenuated total reflectance-Infrared Spectroscopy (ATR-IR) of the solid-state reaction product reveals almost
complete absence of the COOH/O-H stretch in the region 3,600–
2,800 cm 1 (Fig. 2a), in agreement with water formation
during the reaction. Also, the intensity of the carbonyl band due
to either carboxylic group or keto-enol tautomer in the CNT-OH
diminishes significantly with the appearance of the adsorbed
asymmetric CO2 mode at 2,345 cm 1. Compatible with these
conclusions is the large decrease in the bending infrared mode of
the CNTs at 868 cm 1 in the graphene product. The residual
intensity is attributed to the unreacted CNTs. These results were
further confirmed using X-ray photoelectron spectroscopic
measurements.
In the C1s X-ray photoemission spectroscopy (XPS) of the
MWCNTs, the signal at 289.2 eV corresponds to the carboxyl
group, whereas the shoulders at 286.1 and 285.6 eV correspond to
the C–O peak in MWCNT-COOH and MWCNT-OH, respectively15,31. Upon grinding, these features diminish in intensity
and the most dominant peak becomes that of C ¼ C at 284.8 eV,
as seen in Fig. 2b. This is a strong evidence in favour of a
condensation reaction taking place between the COOH and the
OH. In addition, according to XPS, oxygen content drops from
0.715% in the unreacted mixture to 0.280% in the observed solid
product (XPS data, Supplementary Table 1). The water formed
comes from the OH of the carboxylic acid and constitutes half the
oxygen of the carboxylic group. The fact that the loss of oxygen
(0.715–0.280 ¼ 0.435%) is appreciably larger than half the
amount of oxygen of the carboxylic group or oxygen of the
hydroxyl group in the unreactive mixture is in agreement with a
graphene, H2O and CO2 reaction, the yield of which is B61%
(Supplementary Table 1). This is comparable with the infrared
data given before as well as with the simulation data presented
later. To provide further evidence in favour of the graphene
product from the solid-state reaction, we performed Raman
spectroscopy of the reactant MWCNTs and that of the solid-state
reaction product (Fig. 2c). All the bands in the product are
upshifted relative to the reacting MWCNTs, whereas the
second-order Raman band (2D) appearing at 2,705 cm 1 is
downshifted as compared with the graphite band at 2,714 cm 1
(Fig. 2c). On the other hand, the 2D band in the product was well
fitted by a sharp and symmetric Lorentzian in agreement with a
few layer graphene-like product (Fig. 2c). The observations of
lower 2D peak position relative to graphite, smaller ID/IG ratio
(0.201) and larger I2D/IG ratio (1.21) for our reaction product are
all in support of the formation of few layer graphene
materials32,33.
Water formation during the solid-state reaction between the
MWCNTs was also established through an in-situ mass spectrometric study of the reaction products. This was performed as
described in Supplementary Note 1. Briefly, the experiment
involved conducting the solid-state reaction in an enclosed
mortar and pestle and sampling of the gases formed directly with
a quadruple mass spectrometer, in the mass range of 1–300 amu.
A blank measurement was done first without any CNTs in order
to estimate the contribution from atmospheric gases and
moisture. MWCNT-COOH and MWCNT-OH (1:1 ratio by
weight) were then taken in the mortar and ground using a pestle.
The gases in the reaction vessel were then allowed into the mass
spectrometer inlet by opening a valve (Supplementary Fig. 1a).
Intensity of the peak at m/z 18 (due to H2O þ ) increased
significantly than in the blank experiment (Supplementary
Fig. 1d). It is to be noted that there was no increase in intensity
of nitrogen and oxygen ion currents. Increase in the H2O þ peak
intensity with no increase in N þ intensity (derived from N2)
clearly shows that this enhancement is due to the water resulting
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8291
–OH
MWCNT
Functionalization
+
Functionalization
Unzipped nanotube
–COOH
Step A
Step B
Step C
H
O
H Water
Δ
O
O
C
CNT-COOH
O
H
O
H
O
C
OH H
CNT-OH
Exothermic reaction
O C O
Carbon dioxide
C–C breaking bonds
Figure 1 | General scheme of the current work. (a) Solid-state synthetic unzipping scheme. (b) Hydrogen bond-mediated proton transfer unzipping
mechanism: (Step A) hydrogen-bond formation and (Step B) fast proton-transfer, are followed by (Step C) water and CO2 as the products of the
exothermic reaction. The released heat can induce the breaking of carbon-carbon bonds (highlighted yellow region) leading to unzipped tubes.
from solid-state condensation reaction between MWCNTs.
No leak of atmospheric air would explain the increase in
H2O þ as that would have resulted in an increase in N þ intensity
as well. Corresponding mass spectral (intensity versus m/z) data
are given in Supplementary Fig. 1b, which also show that only
H2O þ intensity increased after the reaction while N þ and O þ
intensities remained the same. To check whether this increase is
due to the desorption of water vapour that was adsorbed on
MWCNTs, a control experiment was carried out (Supplementary
Fig. 1c). Initially, a blank was measured without MWCNTs.
MWCNTs were then kept in the mortar without grinding for
2 min and gases inside were sampled (after the evacuation step).
Intensities of N þ , O þ and H2O þ were almost the same as that
of the blank. After the mixture of MWCNTs was ground for
20 min, mass spectral measurement clearly showed an increase in
intensity of only H2O þ . The OH and COOH functionalized
MWCNTs were separately ground and no increase in H2O þ was
detected in those cases. This shows that H2O desorption from
MWNTs is not the reason for increased H2O þ intensity. No
increase in CO2 intensity was seen as it appears to be adsorbed
effectively on the resulting graphene (see above).
Reproducibility of the results was confirmed by repeating these
experiments several times with various ratios of the two MWCNT
varieties. Furthermore, the decrease in the oxygen content,
revealed from the XPS measurements, is supported by our mass
spectrometric detection of water. Hence, the in-situ mass
spectrometric experiments unambiguously give evidence for the
solid-state condensation reaction between -COOH and -OH
groups of the functionalized MWCNTs.
This was further supported with differential thermal analysis at
different temperatures, which gave two distinct peaks, the more
intense one occurs at B50 °C, whereas the less intense one occurs
at B110 °C, shown in Supplementary Fig. 2. The peak at 110 °C
is due to the desorption of water. As the energy evolved at lower
temperature is appreciably higher than that due to desorption of
water and in light of the strong CO2/graphene adsorption, the
intense peak is assigned to the desorption of CO2. This
conclusion is compatible with the infrared and XPS data
presented before.
The proposed reaction from above spectroscopic and thermal
measurements is further verified using imaging techniques such
as scanning (SEM) and transmission electron microscopy (TEM).
Figure 2e,f shows low and higher magnification SEM images
revealing the structure of CNTs for the two functionalized
raw materials. The amount of graphene-like material is
negligible in these samples. On the other hand, the product
powder shows predominantly sheet-like structure along with
residues of partially reacted nanotubes (Fig. 2g,h). The image
analyses of SEM images from different regions are used for
calculating the amount of 2D sheets present and the residue of
CNTs. The sheets are randomly distributed with a range of sizes
with approximately 20% being unreacted CNT. To further
confirm the opening of CNTs and the quality of the graphenelike product, TEM imaging was performed. Figure 3a shows
bright-field image of the functionalized CNTs. It clearly shows
CNTs to be multiwalled with an average diameter of 20 nm. The
surfaces of the nanotubes appear disordered, due to heavy
functionalization. Figure 3b shows large sheets of graphene-like
material with smooth edges. The image shows multilayer
structure. It is possible that the graphene flakes formed during
the reaction have coalesced to form larger multilayer graphitic
sheets. For further confirmation of the structure, selected area
diffraction was performed. The result shows the hexagonal lattice
of graphene stacks (shown as inset). Several images were taken for
different samples of the product at different regions. Figure 3c,d
shows products in the intermediate steps of the reaction. Several
TEM images from different regions have been used for
determining the number of layers in the multilayer sheets and
the size of these graphene sheets. Histograms showing the size
and number of layers formed due to the reaction are shown in
Fig. 3e,f.
Kinetics of the reaction (and product formation) was
monitored by measuring the intensity of the 2D Raman band
of the graphene product at different temperatures30,34–36
(Supplementary Fig. 3). Arrhenius plot (detailed data given in
Supplementary Table 2) of ln k versus 1/T (Supplementary
Fig. 3) gives an activation energy of 16.63 kJ mol 1
(3.97 kcal mol 1), a value compatible with the activation
energies reported for solid-state hydrogen bond-mediated
proton-transfer reactions between many organic compounds
such as carboxylic acid/phenol and carboxylic acid/amine
combinations37–44.
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8291
a
b
50
Intensity (arb. units)
OH
Transmitance
CNT-COOH
OH
CNT-OH
CNT-COOH
40
30
CNT-OH
20
10
CNT
bending
500
CO2
C=C
1,000
1,500
C=0
Graphene
Graphene
2,000
2,500
Wavenumber (cm−1)
3,000
3,500
0
4,000
c
282
284
286
288
Binding energy (eV)
292
d
2D
20
15
15
10
CNT-OH
5
CNT-COOH
Graphene
0
2,500
2,600
2,700
Raman Shift (cm−1)
10
D
D+D”
CNT-OH
CN T s
15
3.50
10
5
0
2,600 2,800
Raman Shift
(cm−1)
2,800
with
Blank
D+D’
Ion current (10−7 A)
G
2D
Intensity (arb. units)
20
Intensity (arb. units)
20
Intensity (arb. units)
290
H 2O +
3.25
1.50
N+
1.25
1.00
5
O+
0.75
CNT-COOH
0
500
Graphene
0
Pumping on
1,500
2,000
2,500
1,000
1,500
Time (s)
Pumping stopped
2,000
Sample line open
2,500
CNTs ground
3,000
Raman Shift (cm−1)
e
CNT-OH
f
CNT-COOH
g
Graphene
h
Graphene
Figure 2 | Materials characterization. (a) ATR-IR spectroscopy of the solid-state reaction graphene product after grinding (red) as compared with
MWCNTs starting material MWCNT-COOH (blue) and MWCNT-OH (black). Formation of water in the unzipping process is confirmed by the absence of
the COOH/OH stretch band in the 3,600–2,800 cm 1 region of the graphene product. The inset shows an image of the product of the solid-state reaction
of MWCNT-COOH and MWCNT-OH. The image shows the thin lustrous sheets of the graphene product formed due to the unzipping of the MWCNTs.
These sheets are covered and surrounded with some traces of the unreacted CNTs. (b) High-resolution C1s XPS spectrum of the solid-state reaction
graphene product obtained after grinding (red) as compared with MWCNTs starting material (blue and green). (c) Raman spectroscopy of the solid-state
reaction mixture after grinding (red) as compared with MWCNTs starting material (green and blue). Insets a 2D-band spectrum of the product as
compared with those in the CNTs starting material, and a single-Lorentzian fit of the 2D band in the product. (d) Ion current versus time plots for
N þ , O þ and H2O þ obtained using in-situ mass spectrometric measurements during the solid-state condensation reaction between MWCNTs. SEM image
of the two reactants (e) CNT-COOH, scale bar, 2 mm; and (f) CNT-OH, scale bar, 2 mm. (g,h) The graphene nanosheet product along with residue of CNTs
at two magnifications, scale bar: (g) 5 mm, (h) 1 mm.
CNT unzipping reaction. In light of the above, we suggest that
the CNT unzipping reaction consists of a slow step that brings the
CNTs together through mechanical grinding allowing the COOH
and OH groups to react (Step A in Fig. 1b). Accordingly, this step
is followed by fast proton transfer (Step B in Fig. 1c) from the
carboxylic group to the hydroxyl group to form [MWCNTOH2] þ and [MWCNT-COO] , whose exothermic reactions
4
produce water and CO2 (Step C in Fig. 1d). The energy released
can induce carbon-carbon bond breaking (highlighted yellow
region) leading to unzipped structures.
Owing to the complexity of this process, simulations are
divided into two parts. The first one is related to the estimation
of the energy barriers associated with the chemical reactions
between the CNT functional groups (value and determination of
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8291
Graphene
40
35
30
25
20
15
10
5
0
Percentage of particles
Percentage of particles
CNT
30
20
10
0
1
2
3
4
No. of layers
5
0
200
400
600
Size (nm)
800
Figure 3 | TEM characterization. Bright-field TEM micrograph of the
reactant (a) CNTs, scale bar, 20 nm, with insets showing low-magnification
image, scale bar, 200 nm and another HRTEM image, scale bar, 5 nm.
HRTEM, high-resolution transmission electron microscopy. (b) Graphene
prepared using the current method, scale bar, 200 nm; inset showing
diffraction pattern. The image shows multilayer structure. The selected area
diffraction performed shows the hexagonal symmetry of graphene. The
high-resolution image of these layered structures shows the hexagonal
lattice arrangement of graphene. (c,d) The images of partially unzipped
CNTs with different magnifications. Scale bar: (c) 10 nm, (d) 5 nm.
(e,f) Histogram of number of layers and size, respectively, of the graphene
sheets produced using the current method.
the thermodynamical character, exothermic or endothermic). In
the second part, we have estimated the minimum required energy
(threshold values, using ReaxFF45,46) value to trigger the
unzipping (breaking carbon bonds). The unzipping can produce
partially or totally opened tubes, thus generating the nanoribbons.
The obtained value was then contrasted against the estimated
energy released during the chemical reactions. This approach is
schematically presented on Fig. 4a–c. The barrier results (Fig. 4d)
were obtained through nudged elastic band (NEB) simulations.
NEB is one of the standard methods used to determine energy
barrier height and the chemical pathways of chemical reactions,
assuming the reactants and products are known47–49. Density
functional theory calculations (see Supplementary Methods)
support the experimental interpretation that water and carbon
dioxide are the final products of the reactions between the CNT
functional groups (initial and the final states are presented in
Fig. 4d).
Energy barrier calculations were performed for different CNT
distances (3.5 to 4.0 Å, with stepsize increments of 0.05 Å; see
Supplementary Fig. 5). These values were chosen over a range
where it is expected that reaction (3.5 Å) and no reaction (4.0 Å)
can occur. Some of these curves are shown in Fig. 4d. The slight
differences from the reference values (for instance, 3.88 instead of
3.90) are a consequence of the initial geometry optimization
process. Our results indicate that the lowest energy barrier can
occur for tubes separated by B3.7 Å, with an associated energy
barrier height around 30 kcal mol 1. More detailed information
can be obtained from the Supplementary Movie 1.
From these results, we can conclude that these reactions need
to be energy assisted. The energy needed to overcome the reaction
barriers of CNTs opening can be provided by the exothermic
reaction between the carboxylic acid and alcohol functionalities,
which in turn provides necessary amount of energy for the
cleavage of carbon–carbon bonds and consequently the unzipping
of the CNTs. The main mechanical grinding effect is only to bring
the CNTs closer. Maximizing the contact area through total or
partial axial tube alignment during the grinding, appears to be the
key to maximize the number of reactions to produce totally or
partially unzipped tubes, which is consistent with the available
experimental data. Our results also showed that this reaction is
exothermic, which is in agreement with more refined density
functional theory calculations that indicate that the released
energy is about 25 kcal mol 1 (see Supplementary Methods for
details). This injected energy can result in thermal (increasing the
temperature around the region where the reaction occurred) and/
or mechanical (eventual C–C bond breaking) effects, thus leading
to tube unzipping, as recently demonstrated by molecular
dynamics (MD) simulations50.
In the second step, to estimate the minimum amount of energy
needed to trigger the unzipping process, we have carried out a
systematic MD study. Our models were composed of MWCNTs,
where for simplicity of the inner-layers are kept frozen. This
approach has been proved effective in the unzipping of carbon51
and boron nitride52 tube studies. To mimic the injected energy
generated by the chemical reactions, we used the so-called
heating spot protocol53,54 (LAMMPS Software manual (http://
lammps.sandia.gov)). Using this protocol, we randomly added
non-translational kinetic energy (heat) to selected atoms within a
subregion defined as ‘contact area’ (highlighted strip in Fig. 4e).
The atoms belonging to the ‘contact region’ can be thought as the
ones that would be in contact with an adjacent tube during the
grinding procedure. The investigated injected energy range was
from 1.0 up to 30 kcal mol 1 (the barrier values). These results
are presented in Fig. 4f (limited to the range of 1.5 up to
5.0 kcal mol 1). The percentage of C–C broken bonds is shown
for each heating spot value. When the heat transferred to the
system is up to 2.0 kcal mol 1, the amount of broken bonds is
around 0.3%, which is not enough to produce the tube unzipping.
Increasing the heat up to 3.0 kcal mol 1, the number increases to
1.0%, which is enough to create some defects in the tube (red
shaded region—Fig. 4f). When the heat delivered to the system is
higher than 3.5 kcal mol 1, the amount of broken bonds reaches
1.6%, which is enough to break up the tube along its main axial
direction and to produce the unzipping effect (grey shaded
region– Fig. 4f). These results show that only a small fraction
(in this case, only 14%) of the estimated energy released by the
chemical reactions would be enough to provide C–C broken
bonds and to trigger the unzipping process.
Discussion
In conclusion, we have reported, for the first time, ambient solidstate mechano-chemical (through simple mechanical grinding)
reactions between CNT-COOH and CNT-OH resulting in
unzipped nanotubes. The released heat during the process results
in C–C bond breaking, which subsequently leads to CNT
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Entire system
1st step: Reaction
2nd step: C-C bonds breaking
CNT-COOH
CNT-OH
2nd step: Heating breaks C-C bonds
1st step: CNT reactions
C (outer shell)
Initial state
C (inner shell)
H
O
Carbon dioxide
Final state
Distance
Water
Oxygen Carbon Hydrogen
Complete view
Random heating spot
Carbon atom view
1.8
CNTs distances
3.55 Å
3.61 Å
3.68 Å
3.74 Å
3.88 Å
50
28.5
1.6
1.4
% C–C breaking
Δ Energy (kcal mol–1)
100
0
1.2
1
0.8
Random
heating spot
0.6
–50
0.4
–100
1
2
3
4
5
6
7
8
0.2
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
Provided energy (kcal mol–1)
Reaction step
Figure 4 | Simulation of the process. (a) General scheme for the simulated unzipping process; (b) functionalized tubes (CNT-COOH and CNT-OH); (c)
initial configuration for energy barrier reaction calculation and resultant unzipped nanotubes. (d) Reactants (different functionalized tubes) and products
(tubes, water and carbon dioxide) of the simulated reaction and the corresponding energy barrier values for different tube separations.
(e) Schematic view of simulated unzipping process: Initial configuration showing the functionalized tube. For better visualization, the CNT functional
groups were made transparent in the following four representative unzipping steps. The yellow line indicates the region where the heating spot was applied.
(f) The percentage of C–C bond breaking and the corresponding final configuration for different energies of the heating spot.
unzipping. The proposed method can be used as a generic
approach to develop new theoretical and synthetic frameworks
where reactions could be designed and controlled via chemically
modified solid reactants.
different colleagues and collaborators involved in this paper. As for the
environment and temperature, this reaction was done in different environments
under vacuum, different labs (that has controlled humidity and temperature) and
also outside in open air as well in variable humidity, different temperatures and in
different times, seasons and countries (Houston (USA), India (Chennai), Lebanon
(Beirut)).
Methods
Synthesis and reactions of CNTs. MWCNTs) were prepared by using waterassisted chemical vapour deposition (WACVD) on a Si substrate. Si wafer, on
which 10 nm aluminum and 1.5 nm iron layers were deposited via electron beam,
was placed in a quartz tube and heated to 775 °C in Ar/H2 buffer having 15% of
H2-ethylene gas and water vapour were introduced in the quartz tube for 30 min.
Iron impurity from the nanotubes was removed by suspending them overnight
in 2.6 M nitric acid at 120 °C. After that the nanotubes were separated by filtration
using 0.2 mm GTTP Millipore membrane and washed extensively with water and
dried. MWCNT-COOH and MWCNT-OH were prepared from pristine CNTs
according to the procedures in refs 1,3, respectively.
The unzipping reaction was done by grinding equal weights of MWCNTCOOH and MWCNT-OH. In current experiment, two different sizes of mortars
and pestles (made up of Traditional Agate) with radii of 2.5 and 5 cm were used.
Initial experiments are performed using mortar of 5 cm radii and then the same has
been repeated for 2.5 cm radii. The samples were collected at different time
duration (5, 10, 15 and 20 min). Among all these, the 20-min grinding shows best
yield, which is reported in the manuscript. We start observing change in the initial
time itself but as the duration of grinding increases, the yield increases as can be
observed as lustrous sheets are formed during grinding. As for the reproducibility
of the results, the reaction was reproduced in three different labs in three different
countries (Professor Ajayan’s Lab in Houston (USA), Professor Pradeep’s lab in
India (Chennai) and Professor Ahmad Kabbani’s lab in Lebanon (Beirut)) by
6
Materials characterization. Raman spectra were measured using a Renishaw
Raman spectrometer at 514.5 nm excitation. The variable temperature Raman
study was carried out by bringing MWCNT-COOH and MWCNT-OH to same
temperatures before grinding them. Grinding was done in a separate vessel at the
same temperature. All reaction mixtures were quenched to room temperature
before Raman spectra were recorded. 2D line intensities at different temperatures
were normalized to the same 2G line intensities. XPS spectra were measured using
a Surface Science Instrument SSX-100. SEM image was obtained by using FEI
Quanta 400 SEM. TEM image was obtained using a JEOL 2100F TEM. Imaging has
been performed using field emission electron microscope with a low current
density and low accelerating voltage. Mass spectrometric measurements were
carried out using a residual gas analyser (BalzersThermostar with Quadstar 32 bit
software). This instrument is basically a residual gas analyser, equipped with an
electron impact ionization source and a quadruple mass analyser. For measurements, we utilized multiple ion detection modes. In multiple ion detection, we
measured ion current as a function of time.
Simulation methodology. We used the ReaxFF force field with parameter set
from refs 45,46. This parameter set is optimized for structures containing the
elements carbon, hydrogen and oxygen, thus appropriated to our systems. For the
NEB calculations, we considered eight replicas between the initial and final
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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8291
configurations. The ‘spring’ that connects each replica has a constant force of
10 kcal /mol 1 Å 1. The adopted criterion for NEB convergence was energy
differences below 1.0 10 4 kcal mol 1. All the calculations using ReaxFF and/or
NEB were carried out using the LAMMPS code52,53. For the MD simulations of the
CNT unzipping processes, the addition of heat (through the heat spot protocol)
into the system is performed every 1.0 ps during 50 ps. The heat is added to the
atoms belonging to a circular (4 Å radius value). We also tested different circle radii
values, ranging from 3 to 6 Å, but no significant differences in the results were
observed.
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Acknowledgements
This work has been supported by US Department of Defense: US Air Force of Scientific
Research for the Project MURI: ‘Synthesis and Characterization of 3-D Carbon Nanotube
Solid Networks’ Award No. FA9550-12-1-0035. P.A.S.A. acknowledge financial support
from the Brazilian Agencies CNPq, CAPES and FAPESP and also thanks the Center for
Computational Engineering and Sciences at Unicamp for financial support through the
FAPESP/CEPID Grant #2013/08293-7. Work at IIT Madras was supported by a grant
through the Nano Mission, Government of India. Part of work was done while MAK was
a visiting student at IIT Madras. P.M.A thanks IIT Madras for a Distinguished Visiting
Professorship.
Author contributions
M.A.K. and A.T.K. proposed the project. M.A.K. and C.S.T. designed and conducted
experiments. P.A.S.A., G.B. performed and D.S.G. supervised the MD simulation.
A.S., K.R.K., S.O., K.H., Y.G. helped in characterization. A.S., K.R.K. performed and
T.P. supervised the mass spectral measurements. T.P. proposed the mechano-chemical
NATURE COMMUNICATIONS | 6:7291 | DOI: 10.1038/ncomms8291 | www.nature.com/naturecommunications
& 2015 Macmillan Publishers Limited. All rights reserved.
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ARTICLE
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms8291
name for the reaction. M.A.K., C.S.T., R.V., D.S.G., T.P., A.T.K. and P.M.A. analysed the
data and wrote the paper. All authors discussed and revised the final manuscript.
How to cite this article: Kabbani, M. A. et al. Ambient solid-state mechano-chemical
reactions between functionalized carbon nanotubes. Nat. Commun. 6:7291
doi: 10.1038/ncomms8291 (2015).
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